Solvent extraction techniques are used to separate uranium and plutonium from fission products and impurities during the reprocessing of irradiated nuclear fuel elements. The fuel elements are dissolved in nitric acid to provide an aqueous feed which is introduced at the top of an extractor column. An organic solvent is introduced at the bottom of the column and the aqueous and organic phases flow in opposite directions through the column and are brought into intimate contact with each other. A packed or plated column can be employed with the lighter organic phase flowing up the column to mix with the heavier aqueous phase flowing down the column. The column can be pulsed to reduce its height and to increase the efficiency of the mixing and extraction.
In one type of column, aqueous raffinate leaves the bottom of the column and solvent containing uranium and plutonium is drawn off at the top of the column. The level of the interface at the bottom of the column between the admixed aqueous and solvent phases and the aqueous phase is maintained by regulating and controlling the flow-rate of the aqueous raffinate leaving the bottom of the column. It is known to effect such flow control by passing the raffinate into a vessel containing a weir, the head of liquid flowing through the weir and hence liquid flowrate being controlled by fine adjustments to the vessel internal pressure. However, the sensitivity and degree of control possible with a weir is very limited.